Support for gas-phase oxidation catalyst and process for its production, gas-phase oxidation catalyst, and process for producing acrylic acid

ABSTRACT

A support for a gas-phase oxidation catalyst, the support including a solid acid, of which acid strength (H 0 ) meets an inequality: −5.6≦H 0 ≦1.5; a gas-phase oxidation catalyst including the above support and a complex oxide containing molybdenum and vanadium as essential components, the complex oxide being supported on the support; a process for producing acrylic acid by gas-phase catalytic oxidation of acrolein with molecular oxygen, the process including carrying out the gas-phase catalytic oxidation in a presence of the above gas-phase oxidation catalyst; and a process for producing the above support, the process including controlling an acid strength (H 0 ) of a solid acid so as to meet an inequality: −5.6≦H 0 ≦1.5 by adjusting a calcination temperature in a preparation of the solid acid contained in the support.

BACKGROUNDS OF THE INVENTION

1. Field of the Invention

The present invention relates to a support for a gas-phase oxidationcatalyst and a process for its production, a gas-phase oxidationcatalyst, and a process for producing acrylic acid.

2. Description of the Prior Art

Gas-phase catalytic oxidation is suitable for large-scale production ata lower cost. Therefore, gas-phase catalytic oxidation is now widelyused for the production of variable basic chemicals from petrochemicalfeedstocks on an industrial scale. For example, ethylene oxide isproduced by the gas-phase catalytic oxidation of ethylene, and maleicanhydride is produced by the gas-phase catalytic oxidation of benzene orn-butane. In addition, (meth)acrylic acid is produced by gas-phasecatalytic oxidation starting from propane or propylene, or at least onekind of compound selected from isobutylene, tert-butanol, and methyltert-butyl ether, and providing (meth)acrylic acid through(meth)acrolein.

For these processes of gas-phase catalytic oxidation, there is usuallyused a catalyst comprising an active catalytic component such as acomplex oxide or heteropoly acid, which is supported on an inert supportsuch as alumina or silica.

However, when conventional gas-phase oxidation catalysts using inactivesupports are used for production on an industrial scale, the yield of afinal product is insufficient, and a decrease in catalytic activity isquick, thereby making catalyst life short. Therefore, the conventionalgas-phase oxidation catalysts using inactive supports do not havenecessarily have sufficient satisfactory catalytic performance.

Accordingly, for example, Japanese Patent Laid-open Publication No.8-47641 discloses a method of improving the activity and stability of acatalyst by including a solid superacid, of which acid strength (H₀)meets an inequality: H₀≦−11.93, in a complex oxide catalyst containingmolybdenum and vanadium as essential components when acrylic acid isproduced by the gas-phase catalytic oxidation of acrolein. Further,Japanese Laid-open Patent Publication No. 8-47643 discloses a method ofimproving the activity and stability of a catalyst by including a solidsuperacid, of which acid strength (H₀) of H₀≦−11.93 in a complex oxidecatalyst containing molybdenum and phosphorus as essential componentswhen methacrylic acid is produced by the gas-phase catalytic oxidationor gas-phase catalytic oxidative dehydrogenation of at least onecompound selected from methacrolein, isobutyraldehyde, and isobutyricacid.

However, in such methods using a solid superacid, a process forpreparing the solid superacid is complicated, and the solid superacid isprepared separately. Therefore, these methods become expensive, and theproduction cost of catalyst is inevitably increased.

SUMMARY OF THE INVENTION

Under these circumstances, it is an object of the present invention toprovide a support for a gas-phase oxidation catalyst, the support beingeasy to handle and making easy and simple the preparation of a gas-phaseoxidation catalyst, wherein catalytic performance can be improved whengas-phase catalytic oxidation is carried out using the gas-phaseoxidation catalyst prepared from the support, thereby making it possibleto obtain a final product in a high yield; a process for producing thesupport; a gas-phase oxidation catalyst; and a process for producingacrylic acid.

As a result of the present inventor's extensive studies to attain theabove object, they have found that, in the production of a final productby the gas-phase catalytic oxidation of a starting material compoundwith molecular oxygen, the use of gas-phase oxidation catalyst using asolid acid, as a support, having a specific acid strength lower thanthose of solid superacids can improve catalyst performance, therebymaking it possible to obtain a final product in a high yield, and that asupport containing a solid acid having a specific acid strength caneasily and simply be prepared by adjusting a calcination temperature.These findings have led to the completion of the present invention.

That is, the present invention provides a support for a gas-phaseoxidation catalyst, the support comprising a solid acid, of which acidstrength (H₀) meets an inequality: −5.6≦H₀≦1.5.

In the support of the present invention, the above solid acid maypreferably comprise a (complex) oxide containing at least one kind ofelement selected from aluminum, silicon, titanium, and zirconium. Theterm “(complex) oxide” refers to an oxide or a complex oxide.

The present invention further provides a gas-phase oxidation catalystcomprising a support and a complex oxide containing molybdenum andvanadium as essential components, the complex oxide being supported onthe support.

In the gas-phase oxidation catalyst of the present invention, the abovecomplex oxide may preferably be expressed by formula (1):Mo₁₂V_(a)W_(b)Cu_(c)A_(d)B_(e)O_(x)  (1)wherein Mo is molybdenum; V is vanadium; W is tungsten; Cu is copper; Ais at least one kind of element selected from cobalt, nickel, iron,chromium, lead, and bismuth; B is at least one kind of element selectedfrom antimony, niobium, and tin; O is oxygen; a, b, c, d, e, and x meanatomic ratios of V, W, Cu, A, B, and O, respectively, and meetinequalities: 2≦a≦15, 0≦b≦10, 0<c≦6, 0≦d≦30, and 0≦e≦6, respectively;and x is a numeral value determined by oxidation states of respectiveelements.

The present invention further provides a process for producing acrylicacid by gas-phase catalytic oxidation of acrolein with molecular oxygen,the process comprising carrying out the gas-phase catalytic oxidation ina presence of the above gas-phase oxidation catalyst.

The present invention further provides a process for producing the abovesupport, the process comprising controlling an acid strength (H₀) of asolid acid so as to meet an inequality: −5.6≦H₀≦1.5 by adjusting acalcination temperature in a preparation of the above solid acidcontained in the support.

The support of the present invention has a specific acid strength lowerthan those of solid superacids. Therefore, the support of the presentinvention is easy to handle and makes easy and simple the preparation ofa gas-phase oxidation catalyst. When gas-phase catalytic oxidation iscarried out using the gas-phase oxidation catalyst prepared from thesupport, it is considered that the absorption and desorption of areactant and a product on a catalyst becomes easy because the carrierhas acid points, and complete oxidation is suppressed, and therefore, itbecomes possible to obtain a final product in a high yield while keepinga high conversion rate of a starting material compound. Further, aprocess for producing the above support according to the presentinvention makes it possible to easily and simply control the acidstrength of a solid acid only by adjusting a calcination temperature inthe preparation of the solid acid contained in the support. Thus, thesupport of the present invention and the process for its production, thegas-phase oxidation catalyst, and the process for producing acrylic acidaccording to the present invention can allow the expectation of asignificant reduction in the production cost of basic chemicals, such asacrylic acid, obtained by gas-phase catalytic oxidation. In addition,the present invention is not to be limited by the contents describedabove.

DETAILED DESCRIPTION OF THE INVENTION

The support of the present invention is a support for a gas-phaseoxidation catalyst, the support comprising a solid acid, of which acidstrength (H₀) meets an inequality: −5.6≦H₀≦1.5 (such a solid acid beinghereinafter referred to simply as the “solid acid” in some cases).

The term “support” as used herein refers to a supporting material onwhich an active catalytic component in the gas-phase catalytic oxidationis to be supported, and the support does not necessarily need to beinactive in the gas-phase catalytic oxidation. In fact, when the supportof the present invention is used for a gas-phase oxidation catalyst, itis considered that the absorption and desorption of a reactant and aproduct on the catalyst becomes easy because the support has acidpoints, and complete oxidation is suppressed, and therefore, it makespossible to obtain a final product in a high yield while keeping a highconversion rate of a starting material compound. However, a substancewhich substantially acts as a catalyst for gas-phase catalytic oxidationis an active catalytic component, not a solid acid having a specificacid strength. Thus, in the present invention, a solid acid having aspecific acid strength is referred to as a support and is distinguishedfrom an active catalytic component.

Further, the term “starting material compound” as used herein refers toa compound as a starting material to be subjected to gas-phase catalyticoxidation, and the term “final product” as used herein refers to anobjective product finally obtained by gas-phase catalytic oxidation.

In the present invention, the acid strength (H₀) of the solid acid ismeasured by the method described below in Examples. Further, the phrase“acid strength (H₀) meets an inequality: −5.6≦H₀≦1.5” means that theacid strength (H₀) of the solid acid falls within the above range, thatis, the acid strength (H₀) of the solid acid is not lower than −5.6 andnot higher than 1.5. Therefore, the solid acid may be composed of onekind of solid acid having an acid strength in the above range, or may becomposed of two or more kinds of solid acids having the same acidstrengths (H₀) or different acid strengths (H₀), so long as these acidstrengths (H₀) fall within the above range.

The solid acid used in the present invention is not particularlylimited, so long as it has the specific acid strength. Examples of thesolid acid may include (complex) oxides containing at least one kind ofelement selected from aluminum (Al), silicon (Si), phosphorus (P),titanium (Ti), vanadium (V), zinc (Zn), zirconium (Zr), niobium (Nb),molybdenum (Mo), and tungsten (W). Specific examples of the solid acidmay include alumina, silica, titania, zirconia, silica-alumina,silica-titania, silica-vanadium oxide, silica-zinc oxide,silica-zirconia, silica-molybdenum oxide, silica-tungsten oxide,alumina-titania, alumina-vanadium oxide, alumina-zinc oxide,alumina-zirconia, alumina-molybdenum oxide, alumina-tungsten oxide,titania-zirconia, titania-tungsten oxide, zinc oxide-zirconia, zeolite,and silicon-alminophosphate. These solid acids may be used alone, or twoor more kinds of these solid acids may also be used in combination. Inthese solid acids, (complex) oxides containing at least one kind ofelement selected from aluminum, silicon, titanium, and zirconium arepreferred, and complex oxides containing aluminum and silicon areparticularly preferred.

The solid acid may take the form of a mixture containing two or morekinds of the above (complex) oxides; the form in which the above(complex) oxide(s) is (are) supported on the different kind(s) of theabove (complex) oxide(s); the form of a mixture of the above (complex)oxide(s) and any other solid(s); or the form in which the above(complex) oxide(s) is (are) supported on any other solid(s), so long asthe solid acid taking each of these forms has the specific acidstrength.

The solid acid may be prepared from starting materials containing theconstituent elements of a (complex) oxide(s). For example, the solidacid as a complex oxide containing aluminum and silicon, which isincluded in the above (complex) oxides, can be prepared by, for example,forming a mixture of aluminum powder, alumina sol, and colloidal silicainto a desired shape, followed by calcination. In this case, the totalamount of aluminum powder and alumina sol is not smaller than 60 partsby mass and not greater than 97 parts by mass, preferably not smallerthan 70 parts by mass and not greater than 95 parts by mass, and morepreferably not smaller than 80 parts by mass and not greater than 90parts by mass, relative to 100 parts by mass of the total amount ofaluminum powder, alumina sol, and colloidal silica. The amount ofcolloidal silica to be mixed is not smaller than 3 parts by mass and notgreater than 40 parts by mass, preferably not smaller than 5 parts bymass and not greater than 30 parts by mass, and more preferably notsmaller than 10 parts by mass and not greater than 20 parts by mass,relative to 100 parts by mass of the total amount of aluminum powder,alumina sol, and colloidal silica. The amount of aluminum powder to bemixed is not smaller than 60 parts by mass and not greater than 97 partsby mass, preferably not smaller than 70 parts by mass and not greaterthan 96 parts by mass, and more preferably not smaller than 85 parts bymass and not greater than 95 parts by mass, relative to 100 parts bymass of the total amount of aluminum powder and alumina sol. The amountof alumina sol to be mixed is not smaller than 3 parts by mass and notgreater than 40 parts by mass, preferably not smaller than 4 parts bymass and not greater than 30 parts by mass, and more preferably notsmaller than 5 parts by mass and not greater than 15 parts by mass,relative to 100 parts by mass of the total amount of aluminum powder andalumina sol. The calcination temperature may preferably be not lowerthan 600° C. and not higher than 1,300° C., more preferably not lowerthan 650° C. and not higher than 1,200° C., and more preferably notlower than 700° C. and not higher than 1,100° C. The calcination timemay preferably be not shorter than 0.5 hours and not longer than 50hours, more preferably not shorter than 1 hour and not longer than 20hours.

The method of controlling the acid strength of a solid acid is notparticularly limited, so long as it is a method which can control theacid strength of a solid acid in such a manner that the solid acid hasthe specific acid strength, but a method which the present inventorshave found out, that is, a method of adjusting a calcination temperaturein the preparation of the solid acid, is preferred. The acid strength ofa solid acid can also be controlled by using, for example, a method ofchanging the kind and/or ratio of constituent elements of a complexoxide.

The shape of the support is not particularly limited, so long as anactive catalytic component can be supported on the support. The supportmay take any of conventionally well-known shapes such as powder,particle, granule, sphere, lump, pellet, fracture, fiber, needle,column, and plate. The support may have a size appropriately adjustedaccording to the type of usage, although it is not particularly limitedto specific shapes.

The support of the present invention may preferably be used for agas-phase oxidation catalyst in which a complex oxide containingmolybdenum and vanadium as essential components is supported as anactive catalytic component on the support.

In the gas-phase oxidation catalyst of the present invention, the abovecomplex oxide as an active catalytic component may preferably beexpressed by formula (1)Mo₁₂V_(a)W_(b)Cu_(c)A_(d)B_(e)O_(x)  (1)wherein Mo is molybdenum; V is vanadium; W is tungsten; Cu is copper; Ais at least one kind of element selected from cobalt, nickel, iron,chromium, lead, and bismuth; B is at least one kind of element selectedfrom antimony, niobium, and tin; O is oxygen; a, b, c, d, e, and x meanthe atomic ratios of V, W, Cu, A, B, and O, respectively, and meetinequalities: 2≦a≦15, 0≦b≦10, 0<c≦6, 0≦d≦30, and 0≦e≦6, respectively;and x is a numeral value determined by the oxidation states of therespective elements.

In the gas-phase oxidation catalyst of the present invention, the rateof the complex oxide supported on the support is expressed by thefollowing equation:Supported rate (%)=[mass of complex oxide/(mass of complex oxide+mass ofsupport)]×100,and specifically, this rate may preferably be not lower than 5% and nothigher than 70%, more preferably not lower than 10% and not higher than60%, and still more preferably not lower than 15% and not higher than55%.

The gas-phase oxidation catalyst of the present invention may take ashape appropriately selected according to the shape of the support,although it is not particularly limited to specific shapes.

In the present invention, the gas-phase oxidation catalyst in which acomplex oxide containing molybdenum and vanadium as essential componentsis supported on the support can be prepared by any of the methodsusually used for preparing this kind of catalyst. The starting materialto be used for preparing the catalyst is not particularly limited. Therecan be used usually available ammonium salts, nitrates, carbonates,sulfates, hydroxides, and oxides of the respective metal elements. Therecan also be used compounds containing two or more metal elements. Themethod of allowing a complex oxide containing molybdenum and vanadium asessential components to be supported on the support is not particularlylimited, but may be any of the methods usually used. For example, thegas-phase oxidation catalyst of the present invention can be obtained byallowing an aqueous solution, a suspension, or a powder, each containinga starting material, to be supported on the previously prepared supportthrough impregnation, spraying, or evaporation to dryness, followed bydrying, if necessary; and subsequent calcination of the support with thestarting material supported thereon at a temperature of preferably notlower than 300° C. and not higher than 600° C., more preferably notlower than 320° C. and not lower than 550° C., and still more preferablynot lower than 350° C. and not higher than 500° C., for about 1 to 10hours.

The gas-phase oxidation catalyst of the present invention may containany of conventionally well-known reinforcing agents to be added for thepurpose of improving the strength and degree of powdering of thecatalyst.

The gas-phase oxidation catalyst of the present invention is suitablefor the production of acrylic acid by the gas-phase catalytic oxidationof acrolein with molecular oxygen. In this case, a substance whichsubstantially acts as a catalyst of the gas-phase catalytic oxidation isa complex oxide containing molybdenum and vanadium as essentialcomponents, preferably a complex oxide expressed by the above formula(1), not a solid acid having the specific acid strength. Thus, in thepresent invention, by referring to the above complex oxide as an activecatalytic component and the solid acid as a support, they aredistinguished from each other.

Accordingly, the process for producing acrylic acid according to thepresent invention is characterized in that when acrylic acid is producedby the gas-phase catalytic oxidation of acrolein with molecular oxygen,the gas-phase catalytic oxidation is carried out in the presence of thegas-phase oxidation catalyst as described above.

The process for producing acrylic acid according to the presentinvention can be carried out in substantially the same manner as in anyof the ordinary processes for producing acrylic acid by the gas-phasecatalytic oxidation of acrolein, except that the gas-phase oxidationcatalyst as described above is used. Therefore, a reactor, as well asreaction conditions, to be used in the process for producing acrylicacid according to the present invention, is not particularly limited.For example, any of the ordinary reactors such as fixed bed reactors,fluid bed reactors, and moving bed reactors can be used as a reactor. Inaddition, as for reaction conditions, the gas-phase catalytic oxidationmay be carried out by, for example, bringing a mixed gas, as a feed gas,containing: acrolein at an amount of not smaller than 1% by volume andnot greater than 15% by volume, preferably not smaller than 4% by volumeand not greater than 12% by volume; oxygen at an amount of not smallerthan 0.5% by volume and not greater than 25% by volume, preferably notsmaller than 2% by volume and not greater than 20% by volume; steam atan amount of not smaller than 1% by volume and not greater than 30% byvolume, preferably not smaller than 3% by volume and not greater than25% by volume; and an inert gas, such as nitrogen, at an amount of notsmaller than 20% by volume and not greater than 80%, preferably notsmaller than 50% by volume and not greater than 70% by volume, incontact with the gas-phase oxidation catalyst to effect reactionat atemperature of not lower than 200° C. and not higher than 400° C.,preferably not lower than 220° C. and not higher than 380° C., under apressure of not lower than 0.1 MPa and not higher than 1 MPa, preferablynot lower than 0.1 MPa and not higher than 0.8 MPa, and at a spacevelocity (under STP, i.e., standard temperature and pressure) of notlower than 300 h⁻¹ and not higher than 5,000 h⁻¹, preferably not lowerthan 500 h⁻¹ and not higher than 4,000 h⁻¹. As a feed gas, a mixed gascontaining acrolein, oxygen, and an inert gas, such as described above,and besides, a mixed gas obtained by the addition, if necessary, of airor oxygen and steam to an acrolein-containing gas obtained by the directoxidation of propylene, can be used. In practice, unreacted propylene orby-products such as acrylic acid, acetic acid, carbon dioxide, andpropane, contained in the acrolein-containing gas obtained by the directoxidation of propylene, have no harmful effects on the gas-phaseoxidation catalyst of the present invention.

The gas-phase catalytic oxidation under such conditions provides anacrylic acid-containing gas. The resulting acrylic acid-containing gasis then subjected to post-treatment such as collection, dehydration,separation, and purification, usually carried out in any of theprocesses for producing acrylic acid. Thus, acrylic acid is obtained asthe final product.

The support of the present invention, as demonstrated below in Examples,is easy to handle and makes easy and simple the preparation of agas-phase oxidation catalyst. When gas-phase catalytic oxidation iscarried out using the gas-phase oxidation catalyst prepared from thesupport, it becomes possible to obtain a final product in a high yieldwhile keeping a high conversion rate of a starting material compound.

EXAMPLES

The present invention will hereinafter be described more specifically byreference to Examples and Comparative Examples, but the presentinvention is not limited to these Examples. The present invention can beput into practice after appropriate modifications or variations within arange meeting the gists described above and below, all of which areincluded in the technical scope of the present invention.

In the following Examples 1 to 2 and Comparative Examples 1 to 2, someexperiments of producing acrylic acid by the gas-phase catalyticoxidation of acrolein with molecular oxygen were carried out. At thistime, catalyst performance was evaluated by carrying out the gas-phasecatalytic oxidation with catalysts in which various solid acids havingdifferent acid strengths (H₀) are used as supports.

<Measurement of Acid Strength>

The acid strength (H₀) of a solid acid is determined as follows. When asample to be measured is white colored, the sample is immersed inbenzene, to which a benzene solution containing an acid-base indicatorhaving a known pKa value is added, and a color change, to acidic color,of the indicator on the surface of the sample is observed. It is assumedthat the acid strength (H₀) of the solid acid is between the greatestpKa value of the pKa values of the indicators which do not change colorto the acidic color and the smallest pKa value of the pKa values of theindicators which change color to the acidic color. Further, when all ofthe indicators used change color to the acidic color, it is assumed thatthe acid strength (H₀) is lower than the smallest pKa value of the pKavalues of the indicators, and when all of the indicators used do notchange color to the acidic color, it is assumed that the acid strength(H₀) is higher than the greatest pKa value of the pKa values of theindicators. Indicators used for the measurement of an acid strength areas follows. Indicator name (pKa value): benzalacetophenone (−5.6),dicinnamalacetone (−3.0), and 4-benzeneazodiphenylamine (1.5).

When a sample to be measured is not white colored, first, the sample isplaced in a vessel having a gas discharging line and a gas introductionline, and air is sufficiently discharged from the vessel. Then, anammonia gas is introduced into the vessel, and the ammonia is adsorbedon the sample. Then, a sample temperature is increased while dischargingthis ammonia gas, and the ammonia gas discharged at each temperature iscollected by liquid nitrogen and the amount of the collected ammonia permass of the sample is measured. The acid strength (H₀) of the sample isdetermined from the obtained measurement value based on a calibrationcurve which has separately been prepared using solid acids having knownacid strengths (H₀).

<Evaluation of Catalyst Performance>

The catalyst performance was evaluated by the conversion rate ofacrolein and the yield of acrylic acid, both of which are defined by thefollowing equations:Conversion rate of acrolein (%)=(mole number of reacted acrolein/molenumber of fed acrolein)×100Yield of acrylic acid (%)=(mole number of produced acrylic acid/molenumber of fed acrolein)×100

Example 1

First, 75 parts by mass of γ-aluminum powder having an average particlediameter of 2 to 10 μm and 5 parts by mass of methyl cellulose as anorganic binder were put into a kneader, followed by well mixing. Then,to this mixture were added 8 parts by mass (as an Al₂O₃ content) ofalumina sol having an average particle diameter of 2 to 20 nm and 17parts by mass (as a SiO₂ content) of colloidal silica having an averageparticle diameter of 2 to 20 nm, into which water was further put, andthe mixture was well mixed to give an alumina mixture containing silicaadded. Then, this mixture was molded by extraction, followed by dryingand calcination at 1,000° C. for 2 hours, to give a solid acid which wascomposed of a complex oxide in the form of particles having an averageparticle diameter of 7.5 mm. The acid strength (H₀) of the solid acidobtained met an inequality: −3.0≦H₀≦1.5.

To 2 liters of water were added 350 g of ammonium paramolybdate, 96.6 gof ammonium metavanadate, 44.6 g of ammonium paratungstate, and 12.5 gof ammonium dichromate while heating and stirring the water, followed bywell mixing, to give an aqueous solution. Separately, 87.8 g of cupricnitrate and 4.8 g of antimony trioxide were added to 0.2 liters of waterwhile heating and stirring the water. Both solutions were mixed, and theresulting mixed suspension was put into a ceramics evaporator on a waterbath, to which 1.2 liters of the above solid acid was added as asupport, and the mixture was evaporated to dryness while stirring,followed by calcination at 400° C. for 6 hours, to give a gas-phaseoxidation catalyst in which a complex oxide containing molybdenum andvanadium as essential components was supported on the support containingthe solid acid, of which acid strength (H₀) met an inequality:−3.0≦H₀≦1.5. The composition, excluding oxygen, of this catalyst, otherthan the support, was Mo₁₂V_(4.6)Cu_(2.2)Cr_(0.6)W_(2.4)Sb_(0.2) interms of atomic ratios. In addition, the supported rate of the catalystwas 23%.

One liter of the gas-phase oxidation catalyst was filled into astainless reaction tube of 25 mm in inner diameter, and a mixed gascontaining 5% by volume of acrolein, 5.5% by volume of oxygen, 25% byvolume of steam, and 64.5% by volume of nitrogen was introduced into thereaction tube through a gas inlet of the reaction tube at a spacevelocity of 1,500 h⁻¹ (STP) to effect a gas-phase catalytic oxidation.At this time, the reaction temperature was 260° C.

As for the catalyst performance, the conversion rate of acrolein was98.9%, and the yield of acrylic acid was 95.1%.

Example 2

A gas-phase oxidation catalyst was prepared in the same manner asdescribed in Example 1, except that the calcination temperature of asolid acid was changed from 1,000° C. to 700° C. to give a solid acid,and the gas-phase catalytic oxidation was carried out under the sameconditions as those used in Example 1. The results are shown in Table 1.Further, the acid strength (H₀) of the solid acid met an inequality:−5.6≦H₀≦−3.0.

Comparative Example 1

A gas-phase oxidation catalyst was prepared in the same manner asdescribed in Example 1, except that the calcination temperature of asolid acid was changed from 1,000° C. to 1,400° C. to give a solid acid,and the gas-phase catalytic oxidation was carried out under the sameconditions as those used in Example 1. The results are shown in Table 1.Further, the acid strength (H₀) of the solid acid met an inequality:H₀>1.5.

Comparative Example 2

A gas-phase oxidation catalyst was prepared in the same manner asdescribed in Example 1, except that the calcination temperature of asolid acid was changed from 1,000° C. to 500° C. to give a solid acid,and the gas-phase catalytic oxidation was carried out under the sameconditions as those used in Example 1. The results are shown in Table 1.Further, the acid strength (H₀) of the solid acid met an inequality:H₀<−5.6.

TABLE 1 Acid Calcination Conversion Yield of strength of temperaturerate of acrylic support of support acrolein acid H₀ (° C.) (%) (%)Example 1 −3.0 ≦ H₀ ≦ 1.5 1,000 98.9 95.1 Example 2 −5.6 ≦ H₀ ≦ −3.0 70098.7 94.4 Comp. Ex. 1 H₀ > 1.5 1,400 98.4 90.3 Comp. Ex. 2 H₀ < −5.6 50099.0 88.2

As can be seen from Table 1, both Example 1 in which the solid acid, ofwhich acid strength (H₀) met an inequality: −3.0≦H₀≦1.5, was used as asupport, and Example 2 in which the solid acid, of which acid strength(H₀) met an inequality: −5.6≦H₀≦−3.0 was used as a support, exhibitedhigh conversion rates of acrolein and high yields of acrylic acid. Incontrast, both Comparative Example 1 in which the solid acid, of whichacid strength (H₀) met an inequality: H₀>1.5, was used as a support, andComparative Example 2 in which the solid acid, of which acid strength(H₀) met an inequality: H₀<−5.6 was used as a support, exhibited arelatively low yield of acrylic acid. From these facts, it is found thatwhen the solid acid, of which acid strength (H₀) meets an inequality:−5.6≦H₀≦1.5, is used as a support, acrylic acid can be produced in ahigh yield while keeping a high conversion rate of acrolein.

The support of the present invention is easy to handle and makes easyand simple the preparation of a gas-phase oxidation catalyst. Whengas-phase catalytic oxidation is carried out using the gas-phaseoxidation catalyst prepared from the support, catalyst performance isimproved, and as a result, it becomes possible to obtain a final productin a high yield. Further, the process for producing the supportaccording to the present invention makes it possible to easily andsimply control the acid strength of a solid acid contained in thesupport. Thus, the support and the process for its production, thegas-phase oxidation catalyst, and the process for producing acrylic acidaccording to the present invention can significantly reduce theproduction cost of basic chemicals, such as acrylic acid, obtained bygas-phase catalytic oxidation, and therefore, they make a greatcontribution to the production fields and application fields of thesebasic chemicals.

1. A gas-phase oxidation catalyst comprising a support comprising asolid acid, of which acid strength (H₀) meets an inequality:−5.6≦H₀≦1.5, and a complex oxide containing molybdenum and vanadium asessential components, the complex oxide being supported on the support.2. The gas-phase oxidation catalyst according to claim 1, wherein thecomplex oxide is expressed by formula (1):Mo₁₂V_(a)W_(b)Cu_(c)A_(d)B_(e)O_(x)  (1) wherein Mo is molybdenum; V isvanadium; W is tungsten; Cu is copper; A is at least one elementselected from the group consisting of cobalt, nickel, iron, chromium,lead, and bismuth; B is at least one element selected from the groupconsisting of antimony, niobium, and tin; O is oxygen; a, b, c, d, e,and x mean atomic ratios of V, W, Cu, A, B, and O, respectively, andmeet inequalities: 2≦a≦15, 0≦b≦10, 0<c≦6, 0≦d≦30, and 0≦e≦6,respectively; and x is a numeral value determined by oxidation states ofrespective elements.
 3. The gas-phase oxidation catalyst according toclaim 1, wherein the solid acid comprises a (complex) oxide containingat least one element selected from the group consisting of aluminum,silicon, titanium, and zirconium.
 4. The gas-phase oxidation catalystaccording to claim 1, wherein the rate of the complex oxide supported onthe support is not lower than 5% and not higher than 70%.
 5. Thegas-phase oxidation catalyst according to claim 1, wherein the rate ofthe complex oxide supported on the support is not lower than 10% and nothigher than 60%.
 6. The gas-phase oxidation catalyst according to claim1, wherein the rate of the complex oxide supported on the support is notlower than 15% and not higher than 55%.
 7. A process for producingacrylic acid by gas-phase catalytic oxidation of acrolein with molecularoxygen, the process comprising carrying out the gas-phase catalyticoxidation in a presence of a gas-phase oxidation catalyst as set forthin claim 1.